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Anyone stepping outside in Beijing around midday on June 18 would have
noticed something slightly amiss. The sky was dark enough for cars to
use their headlights, and the air was as thick as a smoky bar just
before last call. After one of the cleanest springs on record, the
Chinese capital's air quality took an unhealthy plunge for the worse.

You wouldn't have known it by the official numbers. The Ministry of
Environmental Protection publishes air quality data online each day at noon, but only
for the previous 24-hour-period. Anyone who checked on the afternoon of June 18
would have seen an air pollution figure that indicated Beijing's skies
were "slightly polluted," referring to the 24 hours before. (See pictures of Beijing's attempt to clean up its air.)

But a glimpse from another monitoring station that gives hourly updates shows
a very different picture. The U.S. Embassy operates a single station
in eastern Beijing that records levels of PM2.5, fine particles considered
particularly dangerous to human health. At noon that same day, the hourly
measure of PM2.5 crept up into the "hazardous" range, according to
U.S. Environmental Protection Agency standards, and hit a maximum
value of 500 for several hours.

The embassy data is published on its own Twitter feed, and while the U.S. doesn't actively
promote the information, it has slowly been getting more attention
from Beijing residents concerned about the city's air quality. "The
U.S. Embassy has an air quality monitor to measure PM 2.5 particulates
on the Embassy compound as an indication of air quality," says Susan
Stevenson, a State Department spokesperson. "This monitor is a
resource for the health of the Embassy community." She cautions that
citywide analyses cannot be done from a single machine, but because
the embassy has the data available, it makes it available to others. (Read "How Twitter Will Change the Way We Live.)

Reporting from one area, the twitter feed is hardly a complete service, but its popularity underscores some shortcomings in the official daily reports. Chinese environmental officials don't regularly release PM2.5 data, and it isn't used to calculate the daily air pollution index. Instead the government figures rely on measurements of larger PM10 particles,
sulfur dioxide and nitrogen dioxide. Several cities including Beijing
and Shanghai are already measuring PM2.5, the state-run China Daily
reported earlier this month, and the government is now considering
what standards to set for the finer particles and ozone.

Ahead of the Beijing Olympics last year environmental officials came
under harsh criticism that they were tweaking pollution data to
artificially raise the number of so-called "blue sky" days when emissions fall
below official targets. American environmental consultant Steven Q.
Andrews accused the government of switching to monitoring stations in
lower pollution areas, changing the makeup of the air pollution index to focus on less prevalent pollutants, and reporting a
disproportionately large number of days with pollution measurements
just below the "blue sky" cutoff. Du Shaozhong, the deputy head of the
Beijing Environmental Protection Bureau, denied the allegations. In a
recent paper, Andrews reported on similar "blue sky" biases in several
other major Chinese cities.

In the past year, the proportion of days just under the "blue sky"
cutoff has decreased in Beijing. Whether that's a sign that the
numbers are more accurate, or merely better gamed, is still unclear. The city's
hot, humid summers and occasional sandstorms mean that air quality can
turn bad with surprising speed. Without real-time reporting, the official data are more a matter
of historical interest. This afternoon the Ministry of Environmental
Protection reported the air pollution index for the 24 hours ending at
noon on Friday was 159, or "slightly polluted." That's still pretty
bad. But had you gone outside you might have thought things were much
worse.